This invention relates to patient monitoring equipment, probe for use therewith, and method of measuring anesthesia based on oesophagal contractions. It has application in monitoring the depth of anaesthesia of patients to whom anaesthetic or sedative drugs are administered. The term anaesthesia is used herein in its broadest sense and is intended to include not only anaesthesia for surgery, but also the lighter levels of anaesthesia or sedation used in critically ill patients receiving intensive care. The term anaesthetic is to be understood accordingly.
The response of individual patients to drugs is highly variable. Especially in the case of anaesthetic drugs an anaesthetist is required to employ a considerable degree of clinical judgement in order to obtain an optimum effect. Clinical anaesthesia is not an "on-off" state but a state of unconsciousness and variable reflex suppression produced by one or more drugs. It is traditional to describe the degree of reflex suppression as the depth of anaesthesia. At present the depth of anaesthesia is judged by the change in various clinical signs produced in response to surgical stimulus. It would be of great assistance if some objective information were available indicating the depth of anaesthesia. Attempts have been made to use indirect measurements of a patient's vital physiological functions such as heart rate, blood pressure and electroencephalogram (EEG) waveforms to indicate depth of anaesthesia. No one of these measurements alone has proved to be a sufficiently reliable index of anaesthesia.
An article by P. Suppan in the British Journal of Anaesthesia, (1972) 44, p. 1263 describes the use of pulse rate as an indicator of depth of anaesthesia, and describes furthermore the use of a feed-back system to automatically control anaesthetic administration. The article also describes the possibility of using blood pressure as an indicator of the depth of anaesthesia, but there is no suggestion of the combined use of the parameters, or any suggestion that combining two or more measurements to produce a "score" can provide a more reliable indication of depth of anaesthesia.
M. Dubuis, D. E. Scott, and T. M. Savege, in an article in Annals Anaesthesia, France (1979) 3, p 215 describe the use of EEG as an indicator of the after effects of anaesthesia.
Electronically processed EEG signals have been employed to monitor the level of electrical activity in the brain during anaesthesia. A review of this and other applications of EEG monitoring is given in Monitoring Cerebral Function (author P. F. Prior, published by Elsevoir (North-Holland Biomedical Press, 1979, Amsterdam).
Finally J. S. Stewart in The Lancet (1969) 1, p 1305 describes a monitoring system for drawing the attention of a clinician to a deteriorating condition of a patient, using a combination of various parameters, such as heart rate, blood pressure, and oxygen tension. There is, however, no suggestion in the Stewart article of the use of a similar system to measure depth of anaesthesia.
We have discovered that the muscular activity in the oesophagus is related to the depth of anaesthesia. During light anaesthesia there is a great deal of smooth muscle activity in the form of periodic contractions. During deep anaesthesia there is little oesophageal smooth muscle activity. We have consistently observed this relationship between oesophageal activity and depth of anaesthesia with most common anaesthetic agents. Changes in oesophageal muscle activity cause corresponding changes in intra-lumenal oesophageal pressure. Thus, by insertion of a balloon-type catheter, or some other suitable pressure probe, into the oesophagus, and measuring the internal pressure in the oesophagus, it is possible to obtain an indication of the depth of anaesthesia.
The pressure changes produced by oesophageal contraction generally last 2-4 seconds and occur at frequencies of up to 4 or 5 per minute during light anaesthesia. Occasionally there are short periods of rapid contractions at rates of up to 15 per minute accompanied by high resting pressures between contractions.
Oesophageal activity has in the past been observed for a variety of purposes, for example N. E. Leatherman in an article in Critical Care Medicine (1978) Vol 6, No 3 p189 describes the use of an oesophageal balloon for measuring intra-pleural pressure in the monitoring of acutely ill patients. However, we are not aware of any proposals for using measurements of oesophageal activity to monitor depth of anaesthesia.
We have also discovered that, whether or not oesophageal contractions are used as a measure of the degree of anaesthesia, increased reliability in the quantification by the anaesthetist of depth of anaesthesia can be obtained if a plurality of different bodily functions are observed, and a score value assigned to each in accordance with certain parameters. The score values may then be summed to produce a total score indicative of the degree of anaesthesia of the patient.
In one aspect of the method of monitoring anaesthesia proposed herein, signals are produced indicative of the contractions in the oesophagus of a patient, and an output is derived from the signal indicative of the degree of anaesthesia of the patient.
According to a first aspect of the invention, there is provided patient monitoring apparatus, comprising a sensor for providing signals indicative of contractions in the oesophagus of a patient, and means for deriving from the said signals an output indicative of the depth of anaesthesia of the patient.
Preferably the output is derived from the rate of generation of signals produced by oesophageal contraction. Accordingly, in one embodiment of this aspect of the invention, there is provided patient monitoring apparatus comprising a sensor for producing signals indicative of contractions in the oesophagus of a patient, and means for producing an output indicative of the rate of occurrence of such contractions. Alternatively, an output may be derived from the amplitude of such signals.
The changes in pressure that occur and which are picked up by the sensor will be not only pressure changes due to oesophageal contraction but also pressure changes resulting from movement of the heart adjacent to the oesophagus and changes in intrathoracic pressure due to ventilation of the lungs. The pressure changes produced by ventilation and movement of the heart are relatively small and are usually less than 20 mm Hg, typically about 10 mm Hg, when measured with a balloon-type catheter of the type hereinafter described. In contrast oesophageal contractions produce changes in intra-lumenal pressure which may be as high as 100 mm Hg and are usually in excess of 20 mm Hg. In order therefore to eliminate signals arising out of activity not due to oesophageal contractions it is desirable to set a threshold value of pressure below which no signals are utilised for monitoring purposes. This can be done by providing the apparatus with discriminating means for rejecting signals of less than a desired threshold magnitude. The threshold magnitude is preferably substantially greater than, for example twice as great as the magnitude of signals produced by forced ventilation of the lungs of the patient.
In a further embodiment of this aspect of the invention, there is therefore provided patient monitoring apparatus, comprising a sensor for producing signals indicative of contractions in the oesophagus of a patient, and discriminating means for accepting only signals of greater than a predetermined magnitude.
Furthermore in order to minimise spurious signals arising from irregularities in the pressure waveform, an inhibition period may be provided immediately following each contraction in excess of the threshold. In the inhibition period no contractions are recognised. A convenient value for threshold pressure is 20-25 mm Hg and for the inhibition period a time of from 5 to 10 seconds may be suitable.
The rate of generation of contraction signals can be derived from a measurement of the time intervals between successive contractions. The contractions may not occur at regular intervals and a rate derived directly in this way would fluctuate frequently. A measurement of such time interval could be stored and averaged to overcome this instability. A more stable and representative rate is obtained by providing means for counting the number of oesophageal contractions over a period of time and indicating the mean or average rate.
In carrying out the invention therefore it is convenient to make the period of time over which the contractions are averaged a moving and adjustable time "window". A convenient value for such a time "window" is up to nine minutes, preferably from 3 to 9 minutes.
It is possible to provoke oesophageal contractions. These provoked contractions are similar to spontaneous contractions but can be provoked at a depth of anaesthesia sufficient to suppress spontaneous contractions. As anaesthesia is deepened the oesophageal response to provocation diminishes. Thus the provision of means for provoking oesophageal contractions allows deeper levels of anaethesia to be monitored.
According to yet a further embodiment of this aspect of the invention, there is therefore provided patient monitoring apparatus, comprising means for stimulating contractions in the oesophagus of a patient and a sensor for producing signals indicative of contractions in the oesophagus of the patient.
The amplitude of the provoked oesophageal response is, in part, related to the depth of anaesthesia. Thus in addition to the rate of oesophageal contractions, the amplitude of the provoked response may be used as a guide to the depth of anaesthesia. Oesophageal contractions may be provoked by the application of a mechanical or electrical stimulus to the oesophagus or contiguous structures for example the pharynx, larynx or trachea. A convenient means of provoking oesophageal contractions is an air or liquid filled inflatable balloon inserted into the trachea or more preferably, the oesophagus.
Since the equipment described above provides an indication of the depth of anaesthesia it is possible to use the indication obtained to control a drug delivery system to achieve a desired depth of anaesthesia.
According to the invention in another aspect anaesthetic control equipment comprises a sensor for providing signals indicative of contractions in the oesophagus of a patient, for example a probe adapted to be inserted into the oesophagus of the patient and means for controlling the delivery of anaesthetic drugs to the patient in accordance with a parameter of the signals so obtained to achieve a desired depth of anaesthesia.
A suitable parameter is the rate of generation of such signals, preferably averaged over a period of time. However, an amplitude measurement of such signals may be used, particularly where provoked contractions are being measured.
It will be appreciated that the anaesthetic control equipment set out above operates as an automatic closed-loop control system.
As mentioned above we have also discovered that, whether or not oesophageal contractions are used as a measure of the degree of anaesthesia, increased reliability in the quantification by the anaesthetist of depth of anaesthesia can be obtained if a plurality of different bodily functions are observed, and a score value assigned to each in accordance with certain parameters, the score values then being be summed to produce a total score indicative of the degree of anaesthesia of the patient.
According to a further aspect of the invention, there is provided a method of monitoring the degree of anaesthesia or sedation of a patient, which method comprises assigning a score value to each of a plurality of different bodily functions, the said score values being indicative of a depth of anaesthesia or sedation, and summing the score values to obtain a total score indicative of the degree of anaesthesia or sedation of the patient.
The invention also provides patient monitoring apparatus, which comprises means for assigning a score value to each of a plurality of different bodily functions of a patient, the said score values being indicative of a depth of anaesthesia or sedation, means for summing the score values, and means for producing an output from the summed score values indicative of the amount of anaesthetic to be given to the patient.
In carrying out the invention means may be provided for measuring one or more of the said bodily functions and for automatically generating a score value from the measured value. This is particularly suitable for functions such as heart rate and blood pressure. Alternatively the means for assigning a score value to a bodily function may take the form of a keypad, for entering a score value in accordance with a clinical assessment of a selected bodily function. The apparatus of the invention preferably includes means for displaying the resultant score.
Examples of bodily functions that may be measured are EEG activity, cardiac output and oxygen consumption. Specific cardiac function indices that may be measured include blood pressure (normally systolic blood pressure, although diastolic or mean blood pressure may alternatively be used) and heart beat rate. Functions in which clinical assessments may be made and a score value assigned via the keypad are the degree of sweating and the formation of tears. Values of bodily functions which are readily measured by instruments, such as systolic blood pressure and heart beat rate may of course be measured by conventional methods, and score values assigned using the keypad.
Alternatively, or additionally to the above-listed functions, other bodily functions may be measured or assessed.
We have found that because the summed score value relates to a plurality of different bodily functions the score is a much more reliable indication of the depth of anaesthesia than measurement of any one individual function by itself.
Means may be included for recording the total score and if desired the individual score values. It may be convenient to record other associated information, for example the time at which the measurements are taken.
The displayed score can be compared with a desired score in a comparator to derive a score error and the rate of administration of appropriate drugs may then be determined in accordance with the score error to achieve a desired depth of anaesthesia. By providing means for entering an initial rate of administration into the equipment and then modifying this rate by the score error a required rate of administration of a drug may be obtained, displayed and automatically controlled.
As indicated above the score obtained may be used to automatically control the delivery of drugs to a patient.
It is desirable to provide a clock which provides control signals to the measuring means to update the score values at regular intervals and where one or more score value assessments are included it may be desirable to include means for prompting the clinician or anaesthetist to enter his current assessments.